The present invention relates to audio transducers based on electro-magnetic operation and more particularly to such transducers for use with surface-mounted devices (xe2x80x9cSMDxe2x80x9d).
It is quite common to use audio transducers in electronic devices to convert an electrical signal input into sound by producing an acoustic output in response to the input signal. The application of such audio transducers, commonly known as buzzers, can be found in automobile dashboard electronics, appliances and portable electronic products. Particularly with portable devices, such audio transducers are required to be both reliable and compact. Such requirements are mostly likely demanded in the field of portable electronic devices, such as cell phones, PDA or pagers. A buzzer on the cell phone generates the ringing or the key stroke tone, in response to the respective electrical signals.
To be reliable, an ideal electromagnetic audio transducer should be able to withstand stringent and harsh tests such as thermal shock and drop test. By thermal shock, it is meant that the buzzer must not fail when temperature goes from one extreme to anther, such as when an user brings his or her cell phone from inside a car to an air conditioned room. Drop test means generally the device must maintain integrity and electrical contact after being dropped by the user.
To achieve compactness, the buzzer is made into a thin integrated chip, which must be able to fit tightly on the circuit board of the electronic device. This has led to the miniaturization of conventional buzzers using printed circuit boards (xe2x80x9cPCBxe2x80x9d) and made way for SMDs. SMDs can be attached to the PCB of the electronic device without costing additional overall thickness.
Numerous thin buzzers for SMD applications have been disclosed in the art, as well in issued United States patents. One such patent issued to Tajima, U.S. Pat. No. Re. 36,828, disclosed an electronic transducer, where the device has a lead frame formed integrally with an outer case and a drive section within the outer case. The outer case has an opening (Tajima, FIG. 1, 3c) for passing the coil terminals, from the drive section, through to be connected to outer surface of the outer case, as shown in FIGS. 1 and 2 of the Tajima patent. The point of contact between the coil terminals and the part of the lead frame, known as the lands, is applied with solder to secure the contact.
Such approach has some disadvantages. One of them is that since the coil terminals are led out from inside the outer case through the opening, they necessarily become in contact with the opening. During thermal shock, i.e. reliability testing, the plastic material tends to expand such that the opening of the outer case ends up stretching the coil terminals, thus causing the soldered contacts to loosen.
Soldering the contact points as shown in the Tajima patent has another disadvantage. Although the opening and contact are formed in a groove section (Tajima, FIGS. 1 and 2, 3b) of the outer case, the overall thickness of the solder may exceed the clearance provided by the groove, thus forming an uneven surface for the outer case. As can be appreciated by anyone skilled in the art, an uneven plane on the outer surface of a SMD device is bound to create yield and defect problems. On the other hand, the amount of solder is limited by the headroom provided in the groove. In other words, too much solder affects the evenness of the surface, whereas too little solder causes poor connection between the lands and the coil terminals. Further, at the end of the process, the opening and the groove on the outer surface will need to be filled with glue, the application of which will contribute to the thickness and evenness on the outer surface.
Another disadvantage occurs when the lead frame is cut to form four terminals for external connections as illustrated in the Tajima patent in FIG. 5 (21, 23, 25, 27). When the frame is cut to construct the terminals, its tin content is exposed, which makes poor contact.
Therefore, it is desirable to have an audio transducer suitable for SMD connection.
It is also desirable to have an audio transducer with improved connection for the coil terminals.
It is further desirable to have an audio transducer with improved contact terminals from the lead frame.
An electromagnetic audio transducer for SMD applications is disclosed. The transducer comprises a lead frame with external terminals, which are formed into a predetermined shape. The transducer has a case with inside and outside surfaces, where the case is integrated with the lead frame to expose the external terminals at its outside surface. The transducer has solder bases formed by exposing the lead frame at the inside surface of the case. The drive section of the transducer has a coil arranged inside the case, with the coil having coil terminals, and the coil terminals are led to the solder bases for electrical connection at the inside surface of the case.
A transducer constructed in accordance with the present invention will achieve a flatter outside surface for SMD mounting. No glue is needed to close any hole in the outer surface, thus preventing the glue from adding to the overall thickness of the device.